In ancient times, it was common to point a bright red mole on a girl's arm like Bai Lianhua to verify women's chastity, which was called "palace sand". People who don't understand think that "guarding the palace" is to keep the beauty of the sacred side.
According to the Natural History of Jin Dynasty, if you feed the gecko with cinnabar, the gecko will turn red all over. After eating seven kilograms of cinnabar, mash the gecko and dye the limbs of the Virgin with it, so the color will not fade. Only after sexual intercourse will its color fade, so it is called "palace sand" to verify women's "chastity" with this legendary method with excellent effect and simple steps. No matter whether it is true or not, some dynasties put "palace sand" on women who were elected to the palace as a sign of whether they had committed adultery.
When it was circulated among the people, many martial arts novelists used "Zhen Gong Sha" to make an issue of it. It is said that palace sand can only be used to verify the virginity of virgins, and married women are absolutely invalid. In fact, this method was popularized with the rise of Neo-Confucianism in the Song Dynasty.